The present invention relates to wireless communications. In particular, it refers to non-coherent multi-user (MU), multiple-input multiple-output (MIMO) communications especially applicable in high mobility scenarios.
Prior art wireless communications techniques, such as Long Term Evolution (LTE) or IEEE 802.11p systems, are based on the concept of coherent communications in which the received signal is demodulated with the help of pilot signals. Pilot signals are reference signals that are known at both sides of the communication system (i.e. at the transmitter and receiver) to allow estimating the coefficients of the propagation channel. To this end, the pilot signals are either transmitted through a dedicated channel or they are embedded within the information data stream, thus consuming resources that would be otherwise dedicated to information data.
There exist, however, many techniques for non-coherent communication, where the transmitted signals have a particular structure that allows detecting data without knowing the channel coefficients at the receiver side. This means that pilot transmission and channel estimation are no longer necessary. A proper design of the transmitted signals can reach the capacity of non-coherent systems. In this sense, many constellation designs and their detection techniques can be found in prior art, most of them based on unitary space-time matrices. Examples of such techniques can be found, amongst others, in the articles: T. L. Marzetta and B. M. Hochwald, “Capacity of a mobile multipleantenna communication link in Rayleigh flat fading,” IEEE Trans. Inf. Theory, vol. 45, no. 1, pp. 139-157, January 1999; B. M. Hochwald and T. L. Marzetta, “Unitary space-time modulation for multiple-antenna communication in Rayleigh flat-fading,” IEEE Trans. Inf. Theory, vol. 46, no. 2, pp. 543-564, March 2000; M. Beko, J. Xavier, and V. Barroso, “Non-coherent communication in multipleantenna systems: Receiver design and codebook construction,” IEEE Trans. Signal Process, vol. 55, no. 12, pp. 5703-5715, December 2007; I. Kammoun, A. Cipriano, and J. Belfiore, “Non-coherent codes over the Grassmannian,” IEEE Trans. Wireless Commun., vol. 6, no. 10, pp. 3657-3667, October 2007; and Non-Coherent Space-Time Trellis-Coded Modulations for Network-Coded Wireless Relay Communications, US20120183020 A1.
Single-user (SU) multiple-input multiple-output (MIMO) transmission schemes exploit multiple transmit and receive antennas to improve the capacity, reliability and resistance to interferences of wireless communications. In this kind of systems, the communication from the transmitter to each user to be served takes place in orthogonal resources (time, frequency, etc.) that are assigned to each user in a previous phase.
Multi-user (MU) MIMO systems are those where the transmitter sends multiple information streams to multiple users overlapping in the same resource.
In current cellular systems, MU-MIMO communication has been shown to generally improve the overall system performance due to its increased sum data rate (aggregated data rate of all the users) with respect to its SU-MIMO counterpart. Previously proposed non-coherent detection techniques, however, 25 are intended for SU-MIMO communication and their extension to allow MUMIMO operation using non-orthogonal resources is not straightforward and has not been yet addressed. Motivated by the increased data rate of MU-MIMO communications, the current invention enables a MU-MIMO operation in a noncoherent framework.
Even though MU-MIMO communications have the potential to improve the performance of wireless communication systems by providing better data rates, prior art techniques have the problem that its application to vehicular communications, whether it is vehicle-to-vehicle (V2V), vehicle-to-device (V2D) or vehicle-to-infrastructure (V2I), is limited by the high mobility that characterizes this type of scenarios. In particular, channel estimation errors degrade significantly the performance of MU-MIMO systems based on coherent techniques, being especially critical as the number of transmission points and/or antennas increases. High mobility scenarios, such as vehicular communications, suffer frequently from channel estimation errors due to the high variability of the propagation channel. As a result, coherent reception in this type of scenarios generally requires the transmission of a higher number of pilots in order to accurately estimate the propagation channel and limit the negative effects of these errors. This reduces the amount of resources that are available for the transmission of data, and therefore, limits the data rate that can be achieved.
Another drawback of pilot-assisted coherent communications is the pilot pollution problem in dense deployment scenarios. When several transmitters are located close to each other, such as in the case of V2V communications, the pilot signals from different transmitters may interfere with each other. This interference may be severe due to the close proximity between transmitters, thus degrading the performance of the system.
The present invention solves the problems of prior art technique given that no pilot is used (preventing pilot pollution) and, furthermore, no estimation of channel is needed thereby improving the overall performance of the system. In particular, the present invention discloses a method for non-coherent multi user MIMO data transmission that comprises the steps of:
a) estimating the signal-to-noise ratio for each receiver;
b) selecting a power sharing factor for each receiver;
c) encoding information to be sent to each receiver into a symbol for each receiver; and
d) transmitting, by a transmitter, a signal that comprises, at least, the power sharing factors and the symbols for all of the receivers.
In a preferred embodiment, the signal transmitted on step d) is a sum of an arithmetical operation between the symbol and the power sharing factor of all of the receivers. Particularly, the signal transmitted on step d) is:√{square root over (γ1)}x1+√{square root over (γ2)}x2+ . . . +√{square root over (γk)}xk wherein γ is the power sharing factor, x is the symbol and the sub-indexes 1, 2, . . . k correspond to the receivers. Such indexes can be, for example, indexes assigned to the receivers wherein the lowest index corresponds to the receiver with the highest power sharing factor, then, the receivers are ordered by their power sharing factor until, the last receiver (k) corresponds to the receiver with the lowest power sharing factor.
Preferably, the estimation of the signal-to-noise ratio is done at each of the receivers and communicated to the transmitter through a control channel. However, in particular embodiments of the present invention, the estimation of the signal-to-noise ratio is done at the transmitter, e.g., by using a previous signal, i.e., a signal corresponding to a previous communication emitted by the receiver. Such previous signal may be an acknowledgment signal from a previous communication.
In yet another embodiment, the present invention contemplates that the estimation of the signal-to-noise ratio may be performed at the receiver. This estimation may be done by using a previous signal, such as a previous communication emitted by the transmitter or, more preferably, by using an acknowledgment or broadcast signal.
In addition, in step b) the power sharing factors are selected so that the sum of the power sharing factors for all of the receivers does not exceed 1. In a particular embodiment, before performing step d) the transmitter may send to each receiver its corresponding power sharing factor. On the other hand, the present invention discloses a non-coherent multi-user MIMO data reception that comprises the steps of:
i) receiving, from a transmitter the power sharing factor for all receivers;
ii) decode the signal corresponding to the highest power sharing factor;
iii) determining if the signal decoded in step ii) corresponds to the current receiver;
iv) if the decoded signal corresponds to the current receiver, finalize the reception;
v) if the decoded signal does not correspond to the current receiver, proceed with the next power sharing factor and repeat steps iii) to iv).
The present invention also envisages that, in step ii), the signal is decoded, for example, by using a maximum likelihood method.
Preferably, in step i) a consecutive index is determined for each receiver wherein the smallest index corresponds to the highest power sharing factor. Also, the indexes may be organized consecutively by the power sharing factors, 30 so that the highest index corresponds to the receiver with the lowest power sharing factor.
More preferably, in step v) the next power sharing factor corresponds to the next index.
Furthermore, the present invention discloses a system comprising at least a receiver and/or at least a transmitter that executes the above-disclosed methods.